An aerial apparatus of this kind is, for example, a turntable ladder with a bendable articulated arm that is attached to the upper end of a telescopic boom. However, the invention is not limited to fire fighting ladders as such, but also includes similar systems such as articulated or telescopic platforms and aerial rescue equipment. These systems are, in general, mounted on a vehicle such that they are rotatable and erectable.
For example, according to document DE 94 16 367 U1, the articulated arm is attached to the top end of the uppermost element of the telescopic boom and protrudes from the fully retracted telescopic boom so that it can be pivoted at any time regardless of the current extraction length of the telescopic boom. Another example of a ladder with an articulated arm which can be telescopic for itself is disclosed by EP 1 726 773 B1. In still another alternative design, the articulated arm is included in the uppermost element of the telescopic boom so that it can be fully retracted into the telescopic boom, but pivoted from a certain extraction length on up, as disclosed in EP 2 182 164 B1.
Moreover, control devices for turntable ladders, elevated platforms and the like are disclosed in EP 1138868 B1 and EP1138867 B1. A common problem that is discussed in these documents is the dampening of oscillations during the movement of the ladder. This problem is becoming even more important with increasing length of the ladder. It has therefore been proposed to attach sensors for detecting the present oscillation movement at different positions along the telescopic boom. For this purpose, strain gauge sensors are used, also called SG sensors in the following (with SG as abbreviation for “strain gauge”), and an additional two- or three-axis gyroscope attached within the upper part of the telescopic boom for measuring the angular velocity of the upper end of the ladder directly, preferably close to the pivot point of the articulated arm or to the tip of the ladder. A controller is provided for controlling the movement of the aerial apparatus on the basis of signal values that are gained from the SG sensors and the gyroscope. During operation, and especially when an input command for moving the aerial apparatus is passed to the controller, the present oscillation status is taken into account by means of processing the signal values, so that the movement of the ladder can be corrected such that the tip of the ladder reaches and maintains a target position despite the elastic flexibility of the boom.
Existing methods to actively dampen the oscillations of the boom of turntable ladders or similar apparatus are not suitable for and not applicable to relatively large articulated ladders, i.e. ladders with an articulated arm and a maximum reachable height of in particular more than 32 m. For these ladders, due to the length of their boom in relation to their cross section, the spatial distribution of the material must be considered, so that lumped-parameter models based on lumped-mass approximations are not suitable to adequately describe the elastic oscillations of such ladders. Also, not only the fundamental oscillation, but also the second harmonic (and possibly higher harmonics) needs to be actively damped, and the influences of the articulated arm and in particular of changes of the pivot angle need to be considered. Also, other than for ladders up to 32 m, the elastic bending in the horizontal direction and torsion cannot be assumed as independent from each other. Instead, all oscillation modes associated with rotations of the turntable consist of coupled bending and torsional deflections, as will be explained in detail below.
Methods for active oscillation damping and trajectory tracking that consider the fundamental bending oscillations for each the elevation and rotation axis only are known from EP 1138868 B1 and EP1138867 B1, which have already been cited above. These are only applicable to ladders without articulated arm and with a maximum height of up to 32 m, for which only the fundamental oscillation needs to be considered for each axis. An enhanced method for articulated ladders is known from EP 1 772 588 B1, where the flexible oscillations of an articulated ladder are approximated using a lumped-parameter model. The model consists of three point masses that are connected to each other via spring-damper elements. The model, and thus also the subsequently developed oscillation damping control, fail to acknowledge the spatially distributed nature of the boom, so that the coupling of horizontal bending and torsion is not included in the design. Also, higher harmonics are not actively damped, but rather are considered as disturbances, which are filtered using a disturbance observer. The method uses strain gauge (SG) sensors at the lower end of the boom or measurements of the hydraulic pressure of the actuators to detect oscillations. For larger articulated ladders, these measurements are not sufficiently sensitive to measure the second harmonic with adequate signal to noise ratio at all ladder lengths and positions of the articulated arm, which is especially necessary for the ladders considered in the present patent application.
An active oscillation damping that acknowledges the spatial extend of the boom is known from EP 2 022 749 B1. The bending of the boom is modeled using Euler-Bernoulli beam theory with constant parameters, and the rescue cage at the tip of the boom is modeled as rigid body, which yields special dynamic boundary conditions for the beam. Based on a modal approximation of the infinite-dimensional model, the first and second harmonic oscillation are reconstructed from the measurements of SG sensors at the lower end and inertial measurements at the upper end of the boom, e.g. a gyroscope that measures rotation rates of the same rotation axis. The oscillation modes are then obtained from the solution of an algebraic system of equations and both are actively damped. In a second approach, a disturbance observer based on a modified model for the first and second harmonic bending motion is proposed, for which the SG sensors are assumed to only measure the fundamental oscillation. Using the observer signals, only the fundamental oscillation is actively damped. The method neither includes the articulated arm nor the coupling of bending and torsion in the horizontal direction. Also, the observer does not take into account the different signal amplitudes of SG sensors and gyroscope.